Add global task configuration defaults

[jsmestad]

TL;DR

Add agent-wide task configuration defaults so settings such as auto-cascade can apply across projects without repeating the same project file. Project configuration remains the higher-precedence override.

Context

Some task settings are user preferences rather than project decisions. Requiring the same .pi/tasks-config.json entry in every project makes those preferences repetitive to maintain.

The auto-cascade control was also labeled “Auto-execute with agents” in /tasks → Settings, while the documentation calls the feature “auto-cascade,” which made the existing control harder to find.

Changes

• Load global defaults from <agent-dir>/tasks-config.json, using pi's getAgentDir() so custom agent directories continue to work.
• Merge <cwd>/.pi/tasks-config.json over global defaults key by key.
• Persist only values that differ from global defaults when the settings menu writes project configuration.
• Rename the settings control to “Auto-cascade agent tasks” to match the documented terminology.
• Document the global configuration path and an autoCascade example.
• Add focused tests for precedence, malformed files, persistence, and save/reload behavior.

Compatibility

When no global config exists, loading and saving behave as before. Existing project configuration remains authoritative for every key it defines, including explicit false values.

Verification

• biome check src/ test/
• tsc --noEmit
• vitest run (178 tests)
• tsc
• git diff --check